So I've been in a Dark Heresy game for a while now and have a Seraphim, but have yet to really utilize the jump pack. Currently I've been put into a position that I can utilize it to really neat effect (I think).

My PC is on top of a roof, so what I told my GM what I want to do it to fall off the roof and use the jump pack to glide safely while also using my flamer from above, swooping in all Angel of Fiery retribution style. but what I cannot seem to find anywhere is what sort of action the guided fall takes. and honestly the whole jump pack setup seems to be missing any sort of in depth rule set that explains anything beyond the general term that you cannot fall to your death, you can double any base movement action, and that you can achieve flyer (12).

RAW?

It's "Flyer(12)" part.

IMHO?

Treating controlled downward-only movement as "assisted" movement could be reasonable, if she starts by jumping from a solid surface. Then the basic action is a Leap (Full action), with Acrobatics|Strength check, only with 2x distance as if in low gravity (and of course there's +10 S from powered armour, and assisted run also doubles possible run up distance, if possible, which gives at least +10 bonus for a longer run-up) on the 1st round, I'd also rule that it's not going to be slower than running before jump, so if a leap is from a run up (it can be impossible e.g. from a narrow ledge), the distance has low cap of running speed (i.e. for AB +3 of a typical human, mov 18). Gradually slowing down to 12 in the following rounds. Plus safe fall.

So with full run-up distance 36 m (run 18 x2) it's +10* (36-4)/4 = +80 for run up. So without extras it's +90 S test right away, and with Acrobatics+20 it's +100 or perhaps +110 (if you count S bonus as relevant for a dash, though normally it's Ag test), and the distance will be DoS m.

Same for grav-chute, except run-up is normal, so it's unimpressive.

On the other end of it - a standing leap is (-10+10=+0) S test, so in non-ideal circumstances walking up to the edge and then going Flyer(12) can be better when you can't easily score 13+ DoS on that test.